## Dissertations, Theses, and Masters Projects

2021

Dissertation

#### Degree Name

Doctor of Philosophy (Ph.D.)

#### Department

Physics

Keith A Griffioen

David Armstrong

Carl E Carlson

Anselm Vossen

#### Abstract

The semi-inclusive deep inelastic scattering process, where an electron scatters off a proton target at high enough energy that the process can be described by the scattering off a single constituent particle, offers targeted access to the internal structure of the nucleon. The process can be described in two phases by parton distribution functions (PDFs), which describe the likelihood of finding a quark or gluon in a particular state inside of the nucleon and then by fragmentation functions (FFs) which describe the likelihood of forming a particular final state particle. One way to study these properties is via the measurement of the beam spin asymmetry, loosely defined as the difference in the production cross section of final state particles from partons with opposite intrinsic angular momenta. The internal physical laws that govern this mechanism are dictated in part by the PDFs and FFs that are of interest. Furthermore, when the final state particles include two hadrons these PDFs are more easily accessed and new and exciting FFs emerge. Azimuthal correlations in the production of hadron pairs in semi-inclusive deep-inelastic scattering provide rich information on nucleon structure. A high precision study of data taken with the newly upgraded CLAS12 detector has been used to extract beam-spin asymmetries for the $\pi^+\pi^-$ channel. The presented data were taken in the fall of 2018 with CLAS12 using a 10.6~GeV longitudinally spin-polarized electron beam delivered by CEBAF incident on a liquid hydrogen target. The wide acceptance and high luminosity of the CLAS12 experiment enable the simultaneous measurement of the scattered electron and multiple final state hadrons with a previously unmatched statistical precision. These data represent the first experimental results obtained with the new CLAS12 system. Beam spin asymmetry amplitudes with respect to the variables $x$, the fraction of longitudinal momentum carried by the struck quark, and $M_{h}$, the invariant mass of the hadron pair, are presented. One extracted asymmetry, sensitive to the azimuthal modulation $\sin(\phir)$, provides the first opportunity to extract the PDF $e(x)$, which provides information about the interaction between gluons and quarks, in a colinear framework. This measurement provides cleaner access to the PDF than alternative methods that also depend on the transverse momentum of the struck quark. A second modulation, sensitive to $\sin(\phih-\phir)$, corresponds to the first ever signal sensitive to the helicity-dependent two-pion fragmentation function $G_1^\perp$. A nonzero signal is reported which describes the dependence of the produced pions on the helicity of the fragmenting quark. A clear sign change is observed around the mass of the $\rho$-meson that appears in model calculations and results from the interference of $s$ and $p$ wave dihadrons. Numerous other asymmetry amplitudes, each sensitive to its own combination of PDFs and FFs, are also extracted.